1. Field of the Invention
The present invention relates to an oxygen sensor for detecting oxygen concentration in an exhaust gas discharged from internal combustion engines, and more particularly to an improved construction of an end portion of such an oxygen sensor.
2 Related Art Statement
Hitherto, there has been known an oxygen sensor as shown in FIG. 4, which comprises a sensor element 21 protected by a metallic cover 23, a metallic housing 22 for mounting the sensor on an outer wall of an internal combustion engine to expose an outer electrode of the sensor element to the exhaust gas to be measured, and at least one lead wire 24 extended through a rubber grommet fitted in an upper open end of the metallic cover 23 at an upper end portion 25 of the sensor for transmitting an oxygen concentration detecting signal from the sensor element 21.
The structure of the conventional upper end portion 25 of the sensor is shown on an enlarged scale in FIG. 5. Referring to FIG. 5, lead terminals 26-1 and 26-2 are longitudinally extended in the upper end portion 25 to electrically connect to the electrode terminals of the sensor element (not shown). Lead wires 27-1 and 27-2 are extended through the rubber grommet 29 to be inserted inside the sensor and connected to the upper end of the lead terminals 26-1 and 26-2, respectively, at connecting portions 28-1 and 28-2 by means of caulking. In a case where the oxygen sensor includes a heater, a lead wire connected to the heater is also extended through the rubber grommet 29 at the upper end portion of the oxygen sensor. The rubber grommet 29 is made of silicon rubber or fluororubber.
Such a rubber grommet 29 is sometimes heated to an abnormally high temperature by radiant heat from exhaust manifolds and exhaust pipes, which are heated to a high temperature when the engine of a vehicle is suddenly stopped after running at a high speed. When the vehicle is running, however, there is no abnormal heating problem since the rubber grommet 29 is cooled by the air.
When the rubber grommet 29 is heated to a temperature higher than 200.degree. C. in particular higher than 230.degree. C., a harmful gas is generated from the rubber grommet 29 and adversely affects the inner electrode of the sensor element in that the partial pressure of the oxygen in the reference air at the inner electrode is varied by the harmful gas to provide an abnormal output. Consequently, the rich voltage and lean voltage are reduced or the inner electrode of the oxygen sensor element is corroded by the harmful gas, thereby reducing the electromotive force generated by the sensor (the rich voltage and lean voltage are reduced).
In the former, the output signal of the sensor is recovered to a normal condition by introducing fresh air into the inside of the metallic cover 23, but in the latter, the output signal of the sensor can not be recovered to a normal 300.degree.even if a fresh air is introduced into the metallic cover, since the electrode has been damaged.
In order to solve the aforementioned problems, it has been proposed to use a resin grommet made of Teflon (Polytetrafluoroethylene, Registered Trade Mark) or polyimide instead of a rubber grommet since such a resin does not generate harmful gases even at a temperature of 300.degree. C. However, a problem occurs that when a grommet made of Teflon or polyimide is heat cycled, a gap is formed between the outer peripheral surface of the grommet of Teflon or polyimide and the inner surface of the metallic cover as shown in FIG. 5. The gap is a result of a difference in coefficient of thermal expansion between the metallic cover and the resin grommet of Teflon or polyimide. The gap allows water to leak into the inside of the sensor thus appealing the partial pressure of oxygen of the reference air upon evaporation of water. As a result, the electromotive force.